Previous implementation of has_ufo, has_vnet_hdr, has_vnet_hdr_len, etc.
did not really probe for virtio-net header support for the netmap
interface attached to the backend. These callbacks were correct for
VALE ports, but incorrect for hardware NICs, pipes, monitors, etc.

This patch fixes the implementation to work properly with all kinds
of netmap ports.

Signed-off-by: Vincenzo Maffione <v.maffione@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>